I-V Curves: A Guide to Measurement
An I-V curve (short for 'current-voltage characteristic curve'), is a graphical representation of the relationship between the voltage applied across an electrical device and the current flowing through it. It is one of the most common methods of determining how an electrical device functions in a circuit. Key properties of electronic devices can also be extracted from the shape and details of the curve, enabling greater insight into their operation.
There are as many different types of I-V curve as there are different types of electronic devices, and their shapes can be very different.
Measuring and Analysing an I-V Curve
An I-V curve measurement is performed by applying a series of voltages to the device. At each voltage, the current flowing through the device is measured. The supplied voltage is measured by a voltmeter connected in parallel to the device, and the current is measured by an ammeter connected in series. An example of this set up is shown in the diagram below.
The measurement can also be performed using a source measure unit, a device capable of simultaneously supplying voltage and measuring current with high accuracy.
The voltages used in an I-V measurement generally depend upon the specific device being tested. For example, a solar cell may be tested between -1 V and 1 V, whilst an LED may use a higher range of 0 V to 10 V.
Sometimes applying a voltage can alter the electronic properties of a device. This can cause the current to change over time, even when the voltage is kept constant. As such, sometimes we require a pause between setting a voltage and measuring the current.
A basic aspect of the operation of an electronic device can be deduced from the position of the curve on the I-V graph. An I-V graph can be split into quadrants around the axes, as shown in the diagram on the right. The quadrants which a device's curve passes through reveals whether it is an active or passive device.
A device with a curve that is only in Quadrants I and III - where both current and voltage have the same polarity (i.e. are both positive or both negative) - is a passive device. Devices like this use the electrical power of the circuit.
A device with a curve in Quadrants II and IV - where current and voltage have opposite polarities - is an active device. An active device creates electrical power whilst in these quadrants.
Examples of I-V Curves
A resistor is one of the simplest electronic devices, and thus has one of the simplest I-V curves. It is a straight line which intercepts the origin and passes through Quadrants I and III - making a resistor a passive device. The current at each voltage is proportional to the resistance following Ohm’s law: I = V / R. Therefore, the gradient of the line is equal to 1 / R, enabling the resistance to be extracted from the I-V curve.
A diode is a semiconducting device which only allows current to flow through it in one direction. This can be seen in the I-V curve. At positive voltages, the curve rises exponentially, indicating that current is free to flow through the device. At negative voltages, the current remains nearly at zero. However, a sufficiently large negative voltage (known as the 'breakdown voltage') will cause the diode to become conductive to negative current. Similar to a resistor, a standard diode is a passive device, operating only within Quadrants I and III.
A solar cell is a device that uses sunlight to produce electricity. In the dark, its behaviour is identical to that of a diode. However, when illuminated, the I-V curve shifts downwards into quadrant IV. This makes a solar cell an active device. When operating in this quadrant, the solar cell supplies electrical power to the circuit that it is connected to. Several key properties of a solar cell can be extracted from its I-V curve.
For example, the open-circuit voltage and short-circuit current are the values at which the I-V curve intercepts the x and y axes respectively. Furthermore, the gradient of the curve at each point can be used to estimate the series and shunt resistances. If you need to characterise solar cell devices, the Ossila Solar Cell I-V Test System is a quick and easy way to get started.
For more information on the measurement and analysis of solar cells, see our solar cell guide.
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I-V Measurements with the Ossila Source Measure Unit
To make it easier to perform I-V measurements with our Source Measure Unit, Ossila has developed the I-V Curve PC software, which enables you to get started with your Ossila Source Measure Unit more quickly. You can download this software for free on our Software & Drivers page.
When used with the Source Measure Unit, the I-V Curve PC software will allow you to:
- Perform I-V measurements between -10 V and 10 V, with voltage step sizes as low as 333 µV.
- Measure low currents with an accuracy of ±10 nA, or high currents up to ±200 mA.
- Customise your measurements by altering the time between applying a voltage and measuring current (settle time).
- Take more advanced measurements using the hysteresis I-V option, which will perform measurements in both forward and reverse directions.
- The software can perform an I-V measurement using one SMU channel, whilst simultaneously supplying a voltage through the other SMU channel - enabling a wider variety of experiments to be performed.
- Measurement data and settings can be saved to .csv files for easy analysis and record keeping. Settings profiles can be saved in the software, making it simpler to repeat measurements.
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